1. Field of the Invention
The present invention relates to a technology for bidirectional data transmission.
2. Description of the Related Art
In a recent data communication system, data (parallel signals) are transmitted via a plurality of parallel signal lines of a bus. A magnetic force generated by a current flowing through a signal line may interfere a signal transmitted thorough an adjacent signal line, and thus, electric induction may be caused, which results in unintended current flow. Such unintended current flow causes signal interference that leads to an erroneous operation. If the signal lines have different lengths, signal delay may occur.
When signals are transmitted via such parallel signal lines for a long distance, for example, a ground (GND) can be provided between the lines to prevent signal interference. However, with the GND, waveform distortion due to an increase in the number of signal lines (wires) and resistance on a signal path may lead to erroneous detection of a signal. In addition, as difference increases in length of the parallel lines, the parallel signals are more likely to arrive at different timings. To avoid the above possibility, a serial communication bus such as a universal serial bus (USB) and an Institute of Electrical and Electronics Engineers (IEEE) 1394 bus can be employed. While inconvenience such as signal delay is not caused in the serial communications, a complicated protocol is required for a serial communication signal. The serial communications may be unsuitable for low-noise communications between terminal devices from the viewpoint of electromagnetic interface (EMI).
To achieve the long distance low-noise communications, differential signaling is employed in which two signal lines are used with respect to one signal line and noise on the signal lines is cancelled based on a difference between signals on the signal lines. The low voltage differential signaling (LVDS) is a signaling technology for low-voltage and low-noise transmission, in which a phase of a signal on one of two signal lines is inverted to halve the amplitude of the voltage. However, the existing LVDS technology can achieve only unidirectional communications (unidirectional transmission) and cannot achieve bidirectional communications (bidirectional transmission) using, for example, a data bus.
A technology called “Bus LVDS (BLVDS)”, in which signals can be transmitted via a plurality of signal lines of a bus using the LVDS technology, has been put into practice. However, in this technology, each signal line achieves unidirectional communications only. Although an LVDS device that achieves bidirectional communications is available, such LVDS device prepares a signal line for communication in each direction. Therefore, to achieve communications using a bus having a plurality of signal lines with such an LVDS device, a large number of devices and large amount of wiring corresponding to the signal lines are necessary, which increases the cost. A signal transmitted via an existing bus such as a CPU bus (hereinafter, “bus signal”) cannot be transmitted by the LVDS. To transmit such a bus signal by the LVDS, for example, an address bus and a control signal are required in addition to a data bus for bidirectional transmission, which further increases the cost.
A typical data communication system of a large-sized copier as an image forming apparatus is explained below. A signal line extends from a printed circuit board (PCB) on which a CPU is installed for a long distance to each load controlled by the CPU. A copier generally includes a reading unit, a writing unit, a main control unit, and a sheet feeding unit. For example, the writing unit needs to include a sensor for detecting the size of a sheet. To detect widths of sheets in sizes of, for example, A4, A3, A2, A1, A0, B4, B3, B2, and B1, at least nine sensors are required to correspond to the number of sheet sizes.
In a case where the main control unit controls the nine sensors, each of the sensors needs three signal lines to be connected to a power source and signals, i.e., a total of 27 signal lines are required to be arranged for a long distance. Because the units of the copier need to control a large number of loads, a large number of signal lines are arranged as a wiring harness (i.e., the size of the wiring harness increases), which increases the cost and can be a cause of noise. Assembling the wiring harness into the apparatus decreases efficiency and increases the amount of wiring, which increases the cost.
To reduce the number of signal lines, a method can be employed of controlling a load of each unit via a PCB that is provided to each unit and that outputs/receives a signal to/from a main control PCB via, for example, serial communications. In this method, the function can be achieved while a signal line is arranged in each unit (not for a long distance). In addition, because only a few number of signal lines are required between the main control PCB and the units for serial communications, the size of the wiring harness can be reduced and efficiency in arranging the wiring harness can be improved.
Although the number of signal lines can be reduced with the above configuration, CPUs need to be installed on the PCBs of the units, respectively, in addition to that of the main control PCB, which increases the steps of developing software and requires a step for examining timing of serial communications. If integrated circuits (ICs) driven via a CPU bus are mounted on the PCBs of the units, respectively, and the main control PCB controls the PCBs via the CPU bus via parallel communications, the time required for developing software for each unit can be reduced. However, for example, an erroneous operation may be caused due to distortion of waveforms of the signal lines, which results from noise, or electromagnetic waves relative to the EMI may be caused.
When two signal lines are used for bidirectional communications by the LVDS as interface standards for transmitting differential signals, two termination resistors are provided to a driver and a receiver for a terminating process performed on the receiver side. To deal with signal reflection caused by mismatching between the termination resistors, one of the termination resistors is disabled.
Communications by the LVDS can be performed using a plurality of packages for bidirectional communications, each of which includes LVDS lines arranged separately as shown in FIG. 14, or by a method of performing point-to-point communications by converting a bus signal of, for example, the BLVDS into an LVDS signal as shown in FIG. 15. Regarding the method shown in FIG. 14, multiple lines such as a bus requires a plurality of packages, which increases the cost, and requires a large space on the PCB. Regarding the method shown in FIG. 15, although the bus signals can be transmitted via a small number of LVDS signal lines and unidirectional communications are achieved, bidirectional communications cannot be achieved. To achieve bidirectional communication with the BLVDS, a plurality of packages is necessary as in the case shown in FIG. 14.
A method for achieving bidirectional communications with an interface (I/F) for communications in one direction is disclosed in, for example, Japanese Patent Application Laid-open No. 2005-18312. In this method, communications in the opposite direction are achieved with a single-ended signal transmitted at a speed lower than that of an LVDS signal (not a differential signal but a signal of, for example, 5 V or 3.3 V with 0 (zero) V as a reference). A differential signal is transmitted with two signal lines to achieve unidirectional transmission, and a single-ended signal is transmitted to achieve communications in the opposite direction. A signal requiring high-speed transmission is transmitted using a differential signal, and a signal not requiring high speed transmission is transmitted using a single-ended signal.
Japanese Patent Application Laid-open No. 2002-254763 discloses an interface technology that achieves high-speed data transmission without a specific extending device. According to the technology, an I/F connecting a printer controller and a printer engine includes a control line capable of bidirectional communications, with which various types of control signals are transmitted, and a data line for transmitting image data from the printer controller to the printer engine. Because no control signals are required for transmitting the image data with this configuration, the image data can be transmitted at high speed.
However, the technology disclosed in Japanese Patent Application Laid-open No. 2005-18312 does not achieve bidirectional communications of the LVDS signals but is for performing unidirectional communications of the LVDS signal, and a single-ended signal is used for realizing communications in the opposite direction. In this case, the speed and type of the single-ended signal is limited and noise is caused. Specifically, this technology cannot be employed for bus signals, and the communications in the direction opposite to that of the LVDS signals is limited to transmission of an auxiliary signal. The technology disclosed in Japanese Patent Application Laid-open No. 2002-254763 also only achieves unidirectional communications of image data via the data lines.